Fluid pressure and velocity sensing apparatus

Abstract

Apparatus for measuring fluid pressure and velocity, and in particular for maintaining an aspirator air velocity constant under varying atmospheric conditions. A precisely controlled air velocity is provided by a reference air source whose frequency is derived from a crystal oscillator. The total air pressure from the aspirator wind tunnel is compared with the total air pressure from the reference air source using a matched thermistor pair technique to convert the pressure difference into an electrical error signal which is used to control the main air source for the aspirator wind tunnel such that the error signal is maintained at zero, thereby maintaining the air velocity in the aspirator constant.

Having thus described my invention, what I claim as new, and desire to secure by Letters Patent is: 
     
       1. In apparatus for measuring the velocity of a first fluid, the combination comprising: means for supplying said first fluid at a velocity and pressure to be determined;   differential pressure sensing means, having a first input for sensing the pressure of said first fluid and a second input for sensing the pressure of a second fluid, for providing a sensed pressure differential between said first and second fluids, with said pressure differential approaching zero as said pressure of said first and second fluids approach being equal;   transducer means responsive to said sensed differential pressure by said differential pressure sensing means for providing an electrical control signal which is indicative of the sensed differential pressure;   an electrically controlled reference pressure source which provides said second fluid to said second input of said differential pressure sensing means at a controlled pressure and velocity, with the velocity and pressure of said second fluid from said reference pressure source being controlled as a function of said electrical control signal; and   means for measuring the velocity of operation of said reference pressure source, with the measured velocity being indicative of the velocity of said first fluid.   
     
     
       2. In an air servo system, the combination comprising: an enclosure to which air is provided;   an electrically controlled air source connected to said enclosure for providing air to said enclosure;   a reference air source;   means connected to said enclosure and said reference air source for sensing the difference in pressure in said enclosure and in said reference air source for deriving a pressure difference signal; and   transducer means responsive to the derived pressure difference signal for providing an electrical error signal to said electrically controlled air source to control the airflow therefrom to said enclosure.   
     
     
       3. In an air servo system, the combination comprising: an air passageway to which air is provided;   an electrically controlled air source responsive to an electrical error signal for providing a controlled airflow to said air passageway at an unknown pressure;   a reference air source for providing air at a reference pressure;   an airflow sensing means connected to sense the pressure differential between the airflow in said reference air source, and the airflow through said air passageway, for producing a pressure differential; and   transducer means responsive to the pressure differential produced in said airflow sensing means for producing said electrical error signal which is indicative of the pressure differential pressure, with said electrical error signal being provided to said electrically controlled air source to control same for providing said controlled airflow therefrom to said air passage way.   
     
     
       4. In an air servo system for an ink jet aspirator, the combination comprising: an air tunnel in said aspirator, having an input and an output;   an electrically controlled air source responsive to an electrical error signal for providing air to the input of the air tunnel of said aspirator at a controlled velocity determined by said electrical error signal, for providing airflow to the output thereof;   a reference air source for providing a reference airflow;   an airflow sensing means for sensing the pressure differential between the airflow from said reference air source and the airflow through said air tunnel in said aspirator, for producing a pressure differential; and   transducer means responsive to the produced pressure differential in said airflow sensing means, and for converting same to said electrical error signal for provision to said electrically controlled air source to control same for providing air therefrom to the air tunnel in said aspirator.   
     
     
       5. In an air servo system for an ink jet aspirator, the combination comprising: an air tunnel in said aspirator, having an air input and an air output;   a controlled air source for providing air to the input of said air tunnel of said aspirator;   a variable speed motor connected to said controlled air source for causing said controlled air source to provide air to said input of said aspirator at a velocity determined by the speed of operation of said variable speed motor;   a servo amplifier having an input, and an output which provides a control signal to said variable speed motor for controlling the speed of operation thereof;   a reference air source for providing a reference flow of air;   a synchronous motor for controlling the airflow of said reference air source;   an oscillator for providing timing signals to control the speed of operation of said synchronous motor;   first and second pressure probes, connected to sense the air pressure in said reference air source and said air tunnel of said aspirator, respectively; and   pressure responsive means for sensing the difference in air pressure between the air pressure sensed by said first and second pressure probes for providing an error signal to the input of said servo amplifier for controlling same to provide said control signal is provided to said synchronous motor.   
     
     
       6. The combination claimed in claim 5, wherein said pressure responsive means includes a matched thermistor pair, being responsive to the pressure difference between said first pressure probe, and the second pressure probe. 
     
     
       7. The combination claimed in claim 6, wherein said matched thermistor pair are included in a Wheatstone bridge which provides said error signal. 
     
     
       8. In apparatus for measuring the velocity of a first fluid, the combination comprising: means for supplying said first fluid at a velocity to be determined;   differential pressure sensing means comprised of a matched thermistor pair, with the first thermistor being responsive to the pressure of said first fluid, and said second thermistor being responsive to the pressure of a second fluid, with the differential pressure sensed by said matched thermistor pair being converted to a control signal which is indicative of the sensed differential pressure;   a reference pressure source comprised of a pump which provides said second fluid to the second thermistor in said differential pressure sensing means at a controlled velocity, with the velocity of said second fluid from said reference pressure source being controlled by said control signal provided from said differential pressure sensing means.   
     
     
       9. In an air servo system, the combination comprising: an air passageway to which air is provided;   a controlled air source responsive to a provided error signal for providing a controlled airflow to said air passageway at an unknown pressure;   a reference air source for providing air at a reference pressure; and   an airflow sensing means connected to sense the pressure differential between said reference air source and the airflow in said air passageway for producing a pressure differential, wherein said airflow sensing means includes a matched thermistor pair, with the first thermistor being responsive to the pressure from said reference air source, and the second thermistor being responsive to the pressure in said air passageway, with the sensed pressure differential being used to control the airflow from said controlled air source to said air passageway.   
     
     
       10. In an air servo system for an ink jet aspirator, the combination comprising: an air tunnel in said aspirator, having an input and an output;   a controlled air source responsive to a provided error signal for providing air to the input of said air tunnel of said aspirator for providing airflow to the output thereof;   a reference air source for providing a reference airflow; and   an airflow sensing means for sensing the pressure differential between the airflow from said reference air source and the airflow through said air tunnel in said aspirator, with said airflow sensing means including a matched thermistor pair which is responsive to the air pressure from said reference air source and the airflow through said tunnel, for providing a pressure differential which is converted to said error signal for provision to said controlled air source to control the velocity of the air therefrom to said air tunnel of said aspirator.   
     
     
       11. In an air servo system for an ink jet aspirator, the combination comprising: an air tunnel in said aspirator having an air input and an air output;   a controlled air source responsive to a provided electrical error signal for providing air at a controlled velocity to the air input of said air tunnel of said aspirator for providing airflow to the output thereof;   a reference air source for providing a reference airflow; and   air pressure differential sensing means for sensing the air pressure differential between the airflow from said reference air source and the airflow from said controlled air source to said air tunnel of said aspirator, including means for converting the sensed air pressure differential to said electrical error signal for provision to said controlled air source for controlling the velocity of the airflow provided therefrom to the air tunnel in said aspirator.